If a bridge receives, in a network provided through bridges, a MAC (media access control) frame whose destination address is a broadcast address, an unknown multicast address or an unknown unicast address, then the bridge broadcasts (or simultaneously sends) the MAC frame to all of the other ports than the port that has received the MAC frame This disadvantageously increase a probability that terminals connected to the network will receive MAC frames unnecessary for the terminals to receive, which causes a problem that links interconnecting the bridges may have their bands compressed due to the unnecessary MAC frames.
Similarly, if a bridge receives a MAC frame which belongs to an unknown VLAN and which has, as the destination address, a broadcast address, an unknown multicast address or an unknown unicast address, then the bridge broadcasts the MAC frame to all of the other ports than the port that has received the MAC frame.
In a network provided through conventional bridges, the above-mentioned problem can be settled by the generic attribute registration protocol (GARP) provided in IEEE standard 802.1D, the IGMP (Internet Group Management Protocol) snooping, or similar protocols.
In the GARP, exchanging attributes with the neighboring bridges enables knowing a neighboring bridge to which a received MAC frame which has a specific multicast address as the destination address or which belongs to a known VLAN should be transferred, which results in unnecessary MAC frames not being sent to any other neighboring bridges. However, this method is disadvantageous in that each of the bridges on the routes has to have a control protocol for this installed and has to exchange information with each other.
Also, in the IGMP snooping, a bridge which is not a supposed recipient snoops into multicast routing information exchanged in an upper layer and knows the direction in which a MAC frame with a specific multicast address should be transferred.
However, this method is disadvantageous in that it is difficult to implement this method in bridges in which a higher-speed transfer is required because a process of snooping into the routing information exchanged in an upper layer is a heavy load to such bridges. Also, it is another problem that a satisfactory effect can not be expected unless all of the bridges on the routes participate in this method as in case of the GARP.
Due to the above-mentioned disadvantages and problems, in the current situation, it is hard to say that such methods as the GARP and the IGMP snooping are widely used as the flooding reduction technique in a network provided through bridges. In particular, there has not been found, among other things, a case in which either of these methods is used in a provider's network which serves a lot of customers and requires higher-speed transfers. The problem of flooding reduction in a bridge-based network becomes more serious in a VLAN (virtual LAN) service provided by a provider for the customers.
If any setting for flood reduction is installed in the bridges on the routes, there will be required the processes of making tests of plural locations of the header of each customer MAC frame and deciding whether to do a control on the basis of a comparison with the setting in order to reduce the flooding according to the setting. Pursuing the flood reduction in a provider's networks, which have to maintain and process the setting information in a logically separated manner in order to serve many customers, will cause trade-off problems that the transfer process itself becomes a heavy load due to, for example, making tests of the customer's VLAN information which would not be usually made while higher-speed transfers are required.
Also, if no flooding reduction measures are taken, unnecessary flooding MAC frames have to be transferred in the edge bridges and the backbone bridges, disadvantageously affecting the other customers' transfer processes. Further, since customer or subscriber lines are generally narrower in the band as compared with backbone lines or customer's premise networks and the band width varies drastically depending on the customer's locations, if flooding of MAC frames occurred in a location of a broader band flows into a location of a narrower band, the communications of the narrower-band location will be disturbed due to unnecessary traffics.
For the above-described reasons, providers have to reduce the flooding of customer frames which pass through the providers' networks without using the GARP, the IGMP snooping or similar protocols and to reduce unnecessary traffics.
The inventors of the present invention have been engaged in the study to solve the above-mentioned various problems in the prior art and have achieved the invention in the course of the study.    Non-patent-related document 1: IEEE Std. 802.1D (for the GARP)    Non-patent-related document 2: RFC2236 IGMP Version 2